1. Field of the Invention
This invention concerns a radiating high-frequency coaxial cable and, more particularly, a radiating high-frequency coaxial cable with openings in the outside conductor, which essentially are slots placed perpendicular to the cable axis.
2. Description of the Prior Art
Radiating high-frequency coaxial cables have been known for a long time because they may be used as antennas, due to the electromagnetic energy escaping through slots formed in the cable's outside conductor. Such cables make communication between mobile receivers, carried for example on vehicles, and a fixed transmitter possible. Looking at the slot configuration over the entire cable length, the cable is essentially a string of series-connected antennas, which create a radiation field in the vicinity of the cable.
As is already known from commonly owned U.S. Pat. No. 5,276,413, a decrease in the intensity of the radiated output takes place along the cable length due to the natural cable attenuation and the radiation. In practice, this means that the system attenuation between a vehicle and the radiating cable increases along the cable length from the point where the high-frequency energy is fed into the cable. To ensure that the mobile receiver's received field strength is at least somewhat constant, the known radiating high-frequency cable disclosed in the above mentioned United States Patent provides compensation for the effect of the line attenuation by means of a special slot configuration. Accordingly the number of slots per period length increases along the cable in accordance with an appropriate rule. As is known from the article "Leaky coaxial cable with length-independent antenna receiving level" in International Wire & Cable Symposium Proceedings 1992, pages 748-756, this measure leads to an especially advantageous configuration for transmission frequencies to above 900 MHz. Since these types of cables are typically used in tunnels, to enable the transmission of messages to moving traffic or the transmission of messages from moving traffic to the outside, it is important for the slot configuration in the outside conductor of the high-frequency coaxial cable to compensate for the effect of the line attenuation over the longest possible length.
In using new techniques of tunnel construction, the length to be spanned by a radiating high-frequency coaxial cable is not easily obtained with the known cable construction methods. In such long cable lengths, to compensate for the increased line attenuation due to the increasing radiation along the cable length, and thereby creating an essentially constant signal level along the cable, slot configurations would be needed in the outside conductor which cannot be accommodated because of space reasons. Thus, an increase in the numbers of slots per length is not possible at the heavily perforated end of the cable for reasons of space. At the lightly perforated end of the cable, one slot per period length is needed to generate the clock pulse in the cable, so that no further "thinning out" can be accomplished there.